This invention relates to the shorting and grounding of enclosed gas-insulated bus bars in general and more particularly to an improved arrangement which permits shorting higher residual generator currents.
Shorting and grounding devices for bus bars are necessary in order to insure that no voltages are present in installations on which work is being performed. Thus, such devices must be designed so that, even if the part being worked upon, and which is shorted to ground, is accidentally connected to voltage-carrying parts, the potential of the portion of the bus bar which is shorted will have essentially no voltage with respect to ground. In the event of an unintentional energizing of such a bus bar, the shorting and grounding device must have sufficient short circuit capacity to insure this condition. When such a device is to be used for short circuiting a generator output lead, it is particularly desirable to have a contact arrangement including an advance contact the purpose of which is to short out the residual currents which might be present so that at the time of closing the main contacts, no current will be flowing and thus no arcing will occur.
A type of shorting and grounding device not including an advance contact but which does include a pot containing a movable contact mounted over an opening in the enclosure around the bus bar is described in U.S. patent application Ser. No. 434,357 filed Jan. 17, 1974 now abandoned. An advance contact arrangement which can be used in an installation of this type is disclosed in U.S. patent application Ser. No. 369,228 filed June 12, 1973 now U.S. Pat. No. 3,855,434. This arrangement which is particularly useful for shorting generator leads utilizes a vacuum switching tube connected to the advance contact at a movably supported contact. The vacuum switching tube is connected in an electrically conducting manner with a movable contact arm and the latter connected with a contact arms of other shorting devices through connecting straps. With this arrangement, the residual current is shorted using the vacuum switching tubes through the connecting straps after the advance contact is inserted into a mating contact on the bus bar. This arrangement operates quite satisfactorily. However, with the planned use of generators having higher power limits than is presently common, for example more than 15 MVA, the residual currents which must be handled become increasingly larger. As a result, the vacuum switching tube shorting device of the aforementioned application can probably not be used in such an application since the necessary increase in the switching capacity of such a vacuum switching tube would necessitate an increase in its dimensions such that it could no longer be built into the movable contact.
As a result, the need for a shorting device for gas-insulated bus bars in which relatively large residual currents must be shorted through the use of advance contacts becomes evident.